1. Field of the Invention (Technical Field)
The present invention is related to miniature systems for harvesting, generating, storing, and delivering energy, and implementation thereof.
2. Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Wireless sensors and networks of wireless sensors will be used to 1) monitor the structural health of buildings, bridges and aircraft, etc.; 2) monitor the environment, such as in domestic and commercial buildings, and military and homeland security installations; and 3) control industrial processes for increased autonomy, as well as for other tasks. These systems will find use in factory automation, process and environmental control, security, medicine, and condition-based maintenance, as well as in defense applications and intelligence gathering. Widespread use of wireless sensors will improve safety, increase security, lower heating, ventilation and cooling (HVAC) costs, and increase manufacturing efficiency.
Such wireless sensor systems will typically: 1) require numerous individual devices (known as nodes or motes) to provide comprehensive monitoring capability; 2) be located in inaccessible places and 3) require long intervals between scheduled maintenance. Periodic maintenance, such as replacing batteries, would clearly increase operating costs (often to prohibitive levels), and could be inconvenient, at best, if it required interruption of a continuous process. For some remote, hostile, or inaccessible locations, any maintenance may be impossible to perform. In the immediate future, energy management and improved battery technologies may mitigate some of these issues, but in the long term there is clearly a need to develop an energy source that can last years with little or no maintenance.
Miniaturized turbines and micro-fuel cells have been proposed as means of meeting long term energy delivery needs for wireless devices. While these systems exploit the high energy density of hydrocarbon fuels, for example, these systems are inherently limited by the need for a means of storing and delivering a consistent fuel supply, as well as advanced thermal management to safely remove waste heat. These challenges can be overcome; however, the plumbing and additional system engineering (also known as the balance of plant) adds considerably to the overall size and complexity of such systems.
There are additional challenges with micro-fuel cells. Most types use hydrogen fuel, as protons diffuse through an electrolyte more easily than other ions. Storing molecular hydrogen presents significant scientific and engineering challenges, and so most systems use other fuels such as hydrocarbons, methanol or formic acid, or natural gas that must be reformed with steam at high temperature (600° C.) to yield hydrogen and CO. These reformers again add engineering complexity and require extensive insulation for both safety and efficient operation. Furthermore, reported data for micro-fuel cells indicate maximum peak power densities on the order of 50mW/cm2 but with a duration of less than 100 ms. These challenges ensure that combustion and micro-fuel cell power systems will be unable to meet the volumetric energy and power densities needed for severely volume-constrained applications.
Energy harvesters that garner ambient environmental energy (such as light, vibrations, etc.) and convert it to electrical energy are attractive solutions for wireless sensors as they do not need to be replaced, recharged or refueled. Of course, they do not function in the absence of ambient energy (analogous to solar cells at night), and so an energy harvesting power supply must be designed to include some kind of energy storage that can provide back-up power in such situations.
Storage of the energy generated may be accomplished using conventional capacitors, which have very limited energy storage capability (E=½CV2, where the capacitance, C, is typically on the order of a few hundred microfarads, and V=3-5V). This approach leaves the system vulnerable to interruptions in the ambient energy source. Although batteries and or supercapacitors have been proposed as alternative storage devices, no design has effectively accommodated the low power available from small energy harvester devices. Conventional battery chargers, for example, will not operate at the low power levels delivered by energy harvesters, and, besides, even in they could, they would waste too much of the input energy. Further, no existing system discloses the use of optimum energy storage elements for different functions (e.g. back-up power, pulse power, etc.).
Finally, a major challenge that faces wireless sensor nodes is the asymmetry of the power demands of sensing, processing, communication and sleep functions—on the order of four orders of magnitude. Because communication functions draw relatively high power levels (typically from tens to a few hundred milliwatts), wireless sensor nodes are designed to communicate infrequently (for example, once a minute to once an hour), reverting to a low-power sleep state to extend battery life. In order to meet high power communications loads, the usual approach is to design a power source large enough to handle the highest power load. Unfortunately, energy harvesting devices and batteries typically have low power densities, and so power sources are typically oversized for most of the operating needs of the system.
There is, therefore, a need for a simple and compact system which combines energy generation via harvesting of ambient energy sources with energy storage to provide back-up power, and deliver high power pulses as needed.